Scientists have used sequence data to model the current and future human threat of a SARS-like virus found in camels in Saudi Arabia.

The team analysed genomic samples from humans and camels to understand the transmission of Middle East respiratory syndrome coronavirus (MERS-CoV), which is frequently found in camels but also infects humans, causing severe illness and sometimes death. Their findings are published in the journal eLife.

MERS-CoV was first discovered in 2012 and has caused more than 2,000 known cases and over 700 deaths, according to the World Health Organisation. Little is known about the full extent of its transmission to humans, because infections are often seen only in the most severe cases.

“So far, traditional approaches such as recording cases of the infection and looking for antibodies in blood samples have been used to study MERS-CoV,” says lead author Gytis Dudas, Postdoctoral Fellow at the Fred Hutchinson Cancer Research Center, Seattle, US. “But virus genome sequence data have not been used to their full potential. In this study, we wanted to use existing MERS-CoV sequence data to explore the evolution of the virus and its transmission between camels and humans.”

The team studied 274 different MERS-CoV genomes: 174 from humans and 100 from camels. They then applied computational models to reconstruct the evolutionary relationships between the different genetic sequences. Unlike previous studies, the model they used accounted for differences in viral population sizes between its two hosts, which allowed the team to estimate which hosts ancestral viruses infected, as well as when viruses had jumped from one host to another.

They found that MERS-CoV is predominantly a camel virus, and that camels are a ‘reservoir host’ where most of the virus’ evolution takes place. By contrast, they found that human outbreaks were short-lived and eventually die out, leading to no further infections in camels.

The genetic data also demonstrated an important seasonal trend, where the chances of human infection with MERS-CoV was significantly higher between April and July, and decreased in the months of August through December. This is thought to coincide with camel calving, where camels’ innate immunity may be low and could allow MER-CoV to sweep through each new camel generation while they are susceptible to infection.

To further understand the risk to humans, the team used a model to predict the ‘basic reproductive number’, which is the average number of individuals infected secondarily by contact with a primary case. Their model predicted a basic reproductive number of less than 1, suggesting that, on average, a person with MERS will not infect at least one other person, which is necessary to continue the cycle of transmission in humans.

“Our results show that MERS-CoV does not appear to present an imminent global threat, but its epidemiology is nonetheless concerning,” concludes senior author Trevor Bedford, Associate Member in the Fred Hutchinson Cancer Research Center’s Vaccine and Infectious Disease Division. “MERS-CoV joins the list of pathogens able to jump species but not able to spread efficiently in the new host, but sustained evolution of the virus in the camel and continued flow into humans provides the conditions for a more transmissible variant of MERS-CoV to emerge. As such, we encourage continued genomic surveillance of MERS-CoV in the camel and from sporadic human cases to rapidly identify such variants.”

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